[ST-70] Noise when heating up

First of all: hi everybody! This is my first post, so I think I should spend some words to present myself.

I'm Nicola from Italy. More than 40 years ago my grampa bought a fully equipped stereo system with radio, record and audio reel players and (above all!) a Dynakit Stereo 70 with a PAS-2 pre-amplifier and two "Esquire" speakers from ElectroVoice.

About ten years ago I retrieved speakers, ST-70 and PAS-2 and started using them. I had no experience with electronic jobs, so I simply cleaned and powered them up. I used them for ten years.

Last winter the ST-70 began to emit a 120Hz hum. I searched solutions on the internet and replaced the can capacitor. The new quad-cap is the only piece changed in over 40 years, everything else is completely original.

I know that I could make a lot of improvements to my Dynakit but I'm not so expert with electronics and I believe that is better not to touch an old thing if it works!

By the way, some days ago my ST-70 started to make a singolar short noise on the left channel only. Each time I switch it on, after 2 or 3 minutes I can hear this noise once. Apart from that, it works well.

I've seen the quick troubleshooting guide on this forum, especially point 13: "One channel cuts out completely or drops in volume after time". I don't know if my problem is connected to that, so I recorded the noise from my ST-70 left channel to have your opinion.

Noise:http://www.4shared.com/audio/D4PD0X6o/ST-70-Noise.html

Waveform screenshot: img38.imageshack.us/img38/1279/st70noise.png

The noise sounds like when you power up something for a few seconds, then you turn it down. So it has an initial peak and a final peak. Between these two peaks, there are 2-4 seconds of "almost" silence: normally the amplifier has a little operating noise. Between the peaks, the operating noise is a bit louder.

In order to be sure that this noise depends only on the amplifier, I tested it with no input (rca disconnected) and with different speakers. On more recent speakers, peaks are less loud, but still exist.The only coupling of the ST-70 with other pieces is the power cable coming from the PAS-2 output.

Thanks in advance for your opinion and for this forum, I'm new here but I feel this is a friendly community!

CheersNicola

Last edited by Nicola on Tue Dec 13, 2011 6:57 am; edited 3 times in total

Thank you for your answer!I've changed tube sides following these steps:1) inverted the 7199 tubes - noise still present on the same channel2) inverted the first couple of EL34 - noise still present on the same channel3) inverted the second couple of EL34 - noise disappeared!

After step 1 and 2, at step 3 I was pretty sure to get the noise on the other channel but... fortunately noise has completely disappeared.

Unfortunately the 7199 tubes are becoming microphonic: hitting them generate a little noise hearable through the speakers. They also sometimes had generated some crackling noise, so I've moved them a little into the plugs and now I get no more hiss/crackle.I think I'll wait to replace them: microphonic problem seems to be not big actually, and I'm not so happy to spend a lot of money to get two of the few 7199 still available online, neither to replace them with 6GH8A...

A 6GH8A is not a drop in replacement for a 7199 tube. The pin outs are different. To use a 6GH8A in place of a 7199 you have to either rewire the socket OR if you want an easier fix, buy two 7199 to 6GH8A adapters ($20 each).

Bob

Last edited by Bob Latino on Wed Nov 23, 2011 12:11 pm; edited 1 time in total

Yes, that's another possibility but as I said in my first post: I would like to mantain my audio set as original as possible.Changing tubes, caps and the board would improve its sound, but remember I'm running it on a couple of vintage speakers from the 1960s that are possibly the bottleneck of my system...Anyway I think it sounds very well, probably better than some other completely-original-never-modded ST-70s in the world.I'm oriented to just resolve one by one the problems that could happen in the future

Nicola wrote:Thank you for your answer!I've changed tube sides following these steps:1) inverted the 7199 tubes - noise still present on the same channel2) inverted the first couple of EL34 - noise still present on the same channel3) inverted the second couple of EL34 - noise disappeared!

After step 1 and 2, at step 3 I was pretty sure to get the noise on the other channel but... fortunately noise has completely disappeared.

Sometimes the act of removing and reinstalling a tube will improve a poor/dirty connection enough to eliminate a noise caused by poor contact. That may have been what happened in your case.

Unfortunately the 7199 tubes are becoming microphonic: hitting them generate a little noise hearable through the speakers.

Don't do that!! You may have a tube that wasn't microphonic before and make it microphonic by tapping on it while hot. As well, most tubes make some noise when struck - especially if they are in a high gain circuit.

ALL tubes are microphonic to some degree. If the only time a tube makes noise is when you tap it it is not excessively microphonic.

They also sometimes had generated some crackling noise, so I've moved them a little into the plugs and now I get no more hiss/crackle.

Clean the pins and sockets!! Re-tension the output tube sockets if needed.

I think I'll wait to replace them: microphonic problem seems to be not big actually, and I'm not so happy to spend a lot of money to get two of the few 7199 still available online, neither to replace them with 6GH8A...

Clean all the tube pins and sockets - and stop rapping on the hot tubes - and I suspect your problem will disappear.

Ahahah Thank you for your accurate answers!I didn't touch tubes too much fortunately, I've just did a quick test and noticed that there's the "microphonic" phenomen in 2 of 7 tubes! And I also concluded that is a negligible problem, because tubes are not "so much microphonic" to capture interferences or other noises.

I'll clean pins and sockets, that's sure! Any good idea to clean sockets, or just spray cleaner inside?

For the record, there is one instance where tapping on heated tubes is permitted, nay even a necessity. That is when you are performing a "short" test with a tube tester. When testing any suspicious tubes or tubes of unknown condition, a "short" test should be performed before proceeding with further testing. A positive indication of a short inevitably means a rejected tube. Proper testing requires tapping on the tube while switching between the elements. I use a pencil with a rubber pumpkin eraser on top (don't ask ) for my "test instrument".

I've cleaned all tubes' contacts. Unfortunately the noise explained in my first post is back But as Greg_M said, now it is on the other channel (where I moved the supposed-bad EL34). I think I'll have to replace that tube.

I've cleaned all tubes' contacts. Unfortunately the noise explained in my first post is back But as Greg_M said, now it is on the other channel (where I moved the supposed-bad EL34). I think I'll have to replace that tube.

EDIT: no more noise now... That tube is kidding me, I'm sure!

Nicola,

It seems you have an intermittent issue - so replace the suspect tube.

BTW, I use pipe cleaners and D5 DeOxit for octal socket cleaning. For 9 pin sockets you can use the tiny brushes they sell at the drug store for cleaning between teeth. Here is one brand: http://www.piksters.com/

Thank you for cleaning suggestions! It's hard to find DeoxIT in Italy, so I've used the only product available in my local hardware store, the CRC 2-26.It did a good job on switches and tubes. Maybe DeoxIT action is more durable...

CRC 2-26 is a versatile, plastic safe, multi-purpose precision lubricant, penetrant and corrosion inhibitor. Its uniqueviscosity allows it to cover more surface area and penetrate deep into the surface of all metals, including steel,copper, brass and aluminum alloys. 2-26 displaces moisture and leaves a thin, long lasting film to protect againstcorrosion.